Three cases of acute flaccid paralysis (AFP) associated with circulating vaccine-derived poliovirus (cVDPV) isolates were reported in the Philippines during March 15–July 26, 2001. The first case-patient, a child aged 8 years from northern Mindanao island (500 miles south of Manila) who had received 3 doses of oral polio vaccine (OPV), had onset of paralysis on March 15. A second child, aged 3 years from Laguna province on Luzon island (60 miles south of Manila) who had received 3 OPV doses, presented with signs of meningitis but no paralysis on July 23. A third child, aged 14 months from Cavite province (25 miles from Manila and 45 miles north of Laguna province) who had received 2 OPV doses, had onset of paralysis on July 26. No patients had traveled outside of their province of residence since birth. Characterization of isolates from the three patients revealed type 1 polioviruses derived from Sabin vaccine strain type 1, with a 3% genetic sequence difference between Sabin 1 vaccine and vaccine-derived poliovirus (VDPV) isolates. The three polioviruses are not identical but are closely related (>99% sequence homology); they also appear to share an identical recombination site with a nonpolio enterovirus in the noncapsid region of the genome.

Following cVDPV outbreaks in the Dominican Republic and Haiti (Hispaniola) during 2000-2001,1 the global polio laboratory network implemented additional testing requirements for all polioviruses under investigation, prospectively and retrospectively. Both an antigenic-based (ELISA) and a molecular-based test (probe hybridization) are used to determine whether a poliovirus is wild or derived from vaccine (i.e., intratypic differentiation [ITD]). Divergent ITD results (one test showing vaccine-derived and the other wild-type virus) for any poliovirus isolate now require genomic sequencing of the suspect isolates. Retrospective testing of >2,000 vaccine-related isolates from AFP cases globally has revealed no additional cVDPVs, although testing results of other isolates in the laboratory network are pending. The cVDPVs from the Philippines were detected after the implementation of new testing requirements for prospective virus investigations.

In response to these cases, the Department of Health in the Philippines (1) enhanced surveillance by active record review for AFP cases in hospitals and other health-care facilities in the affected and neighboring provinces, (2) established surveillance to conduct virologic investigations of aseptic meningitis at major health-care facilities, (3) collected stool samples from healthy contacts of case-patients, (4) conducted field investigations of clustered AFP cases to determine the extent of cVDPV circulation, and (5) assessed polio vaccination coverage in these communities. The investigations have found no unreported cases, although some AFP cases remain under investigation. To interrupt cVDPV circulation, a large-scale mass vaccination campaign with OPV is planned.

Low routine vaccination coverage is one of the most important causes of VDPV. Because the location of the originating events is unknown, the contribution of other factors is difficult to assess; however, a combination of two concurrent events within the virus is necessary for cVDPV emergence: reversion of attenuating mutations to increase neurovirulence, and a presumed increase in transmission characteristics that might be related to recombination with a nonpolio enterovirus. The molecular basis for the second property is not understood.

Wild poliovirus was last reported in the Philippines in 1993,2 and national vaccination rounds were last conducted in the Philippines in 1997 followed by subnational immunization days in 1998 and 1999. Among the areas covered were Cebu, Davao, Manila, and parts of Mindanao; however, coverage did not extend to the three provinces now reporting cVDPV cases. Routine coverage with 3 OPV doses has been approximately 80% nationwide since the early 1990s; however, coverage gaps are likely, particularly in slum areas.

Travelers to the Philippines should ensure that they are vaccinated appropriately against polio according to national recommendations.3